2008-07-13
New Beacon Antenna
Its been a while coming, but I've been battling the winter flu so the antenna project has dragged on a lot longer than I wanted. Part of the delay was waiting for thick magnet wire to arrive from OBA they were doing a stock take and my order took a few days to process. I was happy enough with that as at the time I was in bed crook anyway.
Once I felt better and had collected all the materials, it was a relatively quick exercise to assemble the new antenna. A three meter section of the 15 mm OD Aluminium tube was used for the radiator. A quick lash in the selected position allowed me to measure the capacitance it saw against the balcony railing. The figure was a surprisingly high 34 pF... Some quick calculations said I needed about 60 uH for a base loading coil with enough room to play with assuming my measurements where a bit off. I temporarily tuned out the antenna capacitance using the previous loading inductor and my C-jig, then using my impedance bridge and R-jig I was able to measure the pure antenna resistance at 22 Ohms. This figure was rather disappointing, but it turned out to be a bit too large, the result of significant coupling to my body in the vicinity of the tests.
Some maths, coil winding and bench testing later I had a 62 uH base load inductor, ~900 pF of matching shunt capacitance and a polyvaricon to match the transmitter into the antenna. It turned out the polyvaricons I recently purchased had sufficient voltage rating for this application, unlike the one I previously cooked in the C-jig. The loading inductor was close-wound despite this giving is a somewhat reduced Q. When the coil losses were compared to the grounding system losses the improvement in Q at the expense of coil volume was not really worth it. The coil is already quite large ~2" ID, 5" long and has a pretty good Q, the SRF is beyond 12 MHz.
A 25 mm dowel and a piece of scrap timber gave me the stand-off and mast for attaching the antenna to the balcony railing. A piece of PVC tubing over the timber provided the base insulator (not shown). To hold the radiator square against the mast I bolted two appropriately sized pieces of channel section back to back with stainless hardware and held the works together with stainless steel hose clamps. This was surprisingly effective and quite easy to assemble with no special tools. Hose clamps also held the mast/stand-off to the balcony railing and facilitated the ground connection.
The matching network was boxed up and lashed to the mast with cable ties. This is not intended to be its permanent fixture, but it seems fine as the matching box is fairly light. If I replaced the cable ties with black UV resistant units it might be better. No doubt the matching box itself will suffer from UV exposure, it is just a Sistema Plastics Klip It Meat Keeper box designed for cold-cuts storage. Cabling from the matching box to the antenna is currently just alligator clip leads, these will probably fall apart in the salt air within a week or two and will be replaced with stainless/Aluminium to copper transitions terminated in banana plugs, all suitably weather proofed.
Connection to the transmitter is made with a few metres of RG-58 coax, the transmitter and switchmode PSU being double bagged and placed under the eaves out of the direct weather. I added a "Tune/Operate" switch to the beacon TX which gives a choice between keyed CW or just solid carrier. This is very useful for tuning up the antenna network. There is some interaction between my body and the antenna, generally I need "overtune" a touch, back off and crouch to minimise my capacitive loading and observe the match. Some iteration finds a good setting of the polyvaricon. Initially I used the impedance bridge, but I've found a NE-2 bulb hanging off one of the hose clamps on the radiator works very well for tuning for maximum smoke (which very nearly coincides with 50 Ohms resistive input impedance to the matching box).
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Attachments
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| AMU Circuit Diagram Source | application/postscript | 10.604 kbytes |
Parent article: 80 Metre CW Beacon.